This invention relates to an ignition timing control method for an internal combustion engine, and more particularly to a method of this kind which controls the ignition timing of the engine when the engine is in a cold state.
A conventional method of controllng the ignition timing of an internal combustion engine has been proposed, e.g. by Japanese Patent Publication No. 50-37333, which comprises determining a basic ignition timing as a function of operating parameters of the engine, and correcting the basic ignition timing so as to advance by a correction value depending upon engine coolant temperature when the engine is cold, to thereby improve driveability of the engine in a cold state. However, according to the proposed ignition timing control method, the advance correction value is determined solely as a function of the engine coolant temperature, so that it is held at the same value so long as the engine coolant temperature remains constant, even if there occurs a change in another engine operating parameter such as the magnitude of engine load. As a result, a required advance of the ignition timing cannot be obtained when large output torque of the engine is desired, such as at engine acceleration from starting position. In order to avoid such inconvenience, if the advance correction value depending upon the engine coolant temperature is set to a value suitable for providing an ignition timing advance required for obtaining large engine output torque, such advance correction value provides an excessive advance of the ignition timing when the engine is operating under low load conditions, causing a drop in the exhaust gas temperature, which can result in degraded emission characteristics of the engine if the engine is provided with a catalyst purifying device due to a delayed increase of the catalyst bed temperature. Thus, the conventional method is not perfect enough to accurately control the ignition timing to values optimal for various operating conditions of the engine.